Biomass is made of organic compounds originally produced by absorbing atmospheric CO2 during the process of plant photosynthesis. As long as the original biomass species are reproduced, cyclic follow of carbon dioxide and other forms of carbon that we use as energy or materials in the atmosphere can be realized. Since, theoretically, there is no net addition of CO2 to the atmosphere in this cycle; biomass is considered one of the key sources of renewable energy in the sustainable society. In the past decades the interest to use biomass as energy and resource production has increased, as these may contribute considerably to the growing future energy and material demands. Energy and resource from biomass can additionally avoid the increase of carbon dioxide in the atmosphere and would help to reduce the greenhouse effect.
One of the most convenient methods to utilize biomass is by conversion to a hydrophobic solid with a high energy density. In traditional torrefaction processes, biomass is heated to 200-300° C. at near ambient pressure in the absence of oxygen to remove moisture and cause some carbonization. The torrified biomass has approximately 30% more energy density than the raw biomass. Typically, energy density can increase to about 18-20 MJ/kg (dry basis) from a biomass input of 17 MJ/kg (dry basis). But a significant loss of heating value occurs due to the evaporation of volatile organic matters, which gives a low energy yield. The chemical composition of torrefied wood is comparable to that of peat.
Recently, use of near critical water (or subcritical water) for carbonizing biomass has attracted lots of attention. Subcritical water serves as an excellent reactive medium due to its specific molecular properties. As compared to ambient water, subcritical water is significantly different in its dielectric constant, thermal conductivity, ion product, viscosity, and density. Subcritical water can efficiently solubilize many of the biomass components and react them without interfacial-transport limitations. A large portion of biomass wastes, e.g. from agriculture and food industries, is wet and contains a high amount of water. This wet biomass causes high drying costs if classical pyrolysis or gasification processes are used. The cost can be advantageously avoided by using subcritical or supercritical water.
When a biomass/water mixture is heated to 230-350° C. and 500-3000 psi (subcritical conditions), an insoluble carbon-rich black solid (biochar) and water-soluble products (biocrude) are obtained. This process is generally termed hydrothermal carbonization. The work in this area dates to as early as the first decades of the 20th century, when the first research work on the hydrothermal carbonization of biomass was carried out to understand the mechanism of coal formation. The key focus was on the change in the oxygen/carbon and hydrogen/carbon atomic ratios upon the chemical transformation.
Past experiments have shown that when carrying out hydrothermal carbonization the oxygen/carbon and hydrogen/carbon ratios of carbon-rich solid product can be changed by modifying the treatment conditions. The product formation is the consequence of dehydration, condensation, or polymerization and aromatization reactions. Experiments have also shown that different sugar sources can have an effect on the structure of the solid product.
Current methods for the hydrothermal carbonization of biomass include converting the biomass together with water and at least one catalyst into substances such as coal, oil, and/or like substances of related type in a pressure vessel by temperature and/or pressure elevation. The biomass, water and/or catalyst is fed into the pressure vessel via a controllable inlet orifice, and the temperature and/or pressure conditions in the pressure vessel are controlled in such a manner that the biomass, water and catalyst react with one another and at least one reaction product is taken off via a controllable outlet orifice. To improve the hydrothermal carbonization of biomass, especially with regard to the duration of the conversion process and also the manner in which the process is carried out, the biomass is sometimes treated before and/or during the conversion by means of ultrasound, microwave radiation, or decompression.
In above traditional and new processes, some carbon is lost as water-soluble compounds giving a low energy yield of biochar. What is needed is a method and system for conversion of biomass to biochar that provides a higher biochar yield and reduction in the carbonization temperature.
Accordingly, it can be seen that needs exist for improved methods and systems that efficiently convert biomass to biochar. It is to the provision of these needs, among various others, that the present invention is primarily directed.